1. Field of the Invention The present invention relates generally to nuclear fuel assemblies for nuclear reactors and, more particularly, is concerned with a nuclear fuel rod support grid employing separate attachable spring and dimple support spacers and a method of assembling a nuclear fuel assembly.
2. Description of the Prior Art In most nuclear reactors, the reactor core is comprised of a large number of elongated nuclear fuel assemblies. Conventional designs of these fuel assemblies include a plurality of nuclear fuel rods held in an organized array by a plurality of grids spaced axially along the fuel assembly length and attached to a plurality of elongated control rod guide thimbles of the fuel assembly. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the guide thimbles which extend slightly above and below the ends of the fuel rods.
The grids of the fuel assembly as well know in the art are used to precisely maintain the spacing between the nuclear fuel rods in the reactor core, prevent rod vibration, provide lateral support for the fuel rods, and, to some extent, frictionally clamp and retain the rods against longitudinal movement.
One popular conventional fuel rod support grid design, being illustrated and described in U.S. Pat. No. 4,492,844 to Kobuck et al, includes a multiplicity of interleaved inner and outer belt-shaped straps having an egg-crate configuration forming a multiplicity of cells which individually accept the nuclear fuel rods and control rod guide thimbles. The cells of each grid which accept and support the fuel rods at a given axial location therealong typically use relatively resilient springs and/or relatively rigid protrusions (called dimples) formed directly into the metal of the interleaved straps. The springs and dimples of each grid cell frictionally engage or contact the respective fuel rod extending through the cell and together apply a clamping force directed generally perpendicular to the axial direction of the fuel rod. Additionally, the outer straps are attached together and peripherally enclose the inner straps to impart strength and rigidity to the grid.
However, several drawbacks arise from this particular grid design. Scratches are generated in the axial direction on the outer surfaces of the fuel rods by the springs and dimples when the fuel rods are inserted into the grid cells at the time of assembling of the fuel assembly. Also, when the fuel rods are inserted through the grid cells in assembling the fuel assembly, considerable pushing (or pulling) force is required since the fuel rods are being moved while clamped between the springs and dimples. Further, the fabrication of the straps for the grid with springs and dimples formed thereon is complicated.
Consequently, a need exists for an alternative approach to constructing the fuel rod support grid and assembling the nuclear fuel assembly which will avoid the above drawbacks without presenting new ones in their place.